Switching controller having frequency hopping for power supplies and method therefor

ABSTRACT

A switching controller having frequency hopping is used for reducing the EMI of a power supply. A pattern generator generates a digital pattern code in response to a clock signal. An oscillator generates an oscillation signal for determining a switching frequency of a switching signal. A programmable capacitor coupled to the oscillator modulates the switching frequency in response to the variation of the digital pattern code. An attenuator connected to a voltage feedback loop attenuates a feedback signal. The feedback signal controls the pulse width of the switching signal. A programmable resistor coupled to the attenuator determines an attenuation rate of the attenuator in response to the digital pattern code. The attenuation rate is increased as the switching frequency increases. The pulse width of the switching signal is thus reduced, which compensates the decrease of the switching period and keeps the output power and the output voltage constant.

CROSS REFERENCE

This application is a continuation application of a prior applicationSer. No. 10/898,389, filed on Jul. 23, 2004. All disclosures areincorporated herewith by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a power supply, and more specificallyrelates to the control of a switching mode power supply.

2. Description of Related Art

Power supplies have been used to convert an unregulated power source toa regulated voltage or current. FIG. 1 shows a traditional power supply,in which a PWM controller 10 generates a switching signal V_(PWM) forswitching a transformer 11 via a transistor 20. The duty cycle of theswitching signal V_(PWM) determines the power delivered from an input ofa power source to an output of the power supply. Although the switchingtechnology reduces the size of the power supply, switching devicesgenerate electric and magnetic interference (EMI) that interferes thepower source. An EMI filter 15 equipped at an input of the power supplyis utilized to reduce the EMI. However, the EMI filter 15 causes powerconsumption and increases the cost and the size of the power supply. Inrecent development, many prior arts have been proposed to reduce the EMIby using frequency modulation or frequency hopping. For example,“Effects of Switching Frequency Modulation on EMI Performance of aConverter Using Spread Spectrum Approach” by M. Rahkala, T. Suntio, K.Kalliomaki, APEC 2002 (Applied Power Electronics Conference andExposition, 2002), 17-Annual, IEEE, Volume 1, 10-14, March, 2002, Pages:93-99. However, the disadvantage of the prior art is that frequencymodulation generates undesirable ripple signal at the output of thepower supply. The undesirable ripple signal generated by frequencymodulation could be realized by the following description. An outputpower P_(o) of the power supply is the product of an output voltageV_(o) and an output current I_(o) of the power supply, which is givenby,P _(o) =Vo×Io=η×P _(IN)  (1)

An input power P_(IN) of the transformer 11 and a switching currentI_(P) can be respectively expressed by,$P_{IN} = {\frac{1}{2 \times T} \times L_{P} \times I_{P}^{2}}$$I_{P} = {\frac{V_{IN}}{L_{P}} \times T_{ON}}$Where η is the efficiency of the transformer 11; V_(IN) is an inputvoltage of the transformer 11; L_(P) is a primary inductance of thetransformer 11; T is a switching period of the switching signal V_(PWM);T_(ON) is an on-time of the switching signal V_(PWM).The equation (1) can be rewritten as, $\begin{matrix}{P_{O} = {\eta \times \frac{V_{IN}^{2} \times T_{ON}^{2}}{2 \times L_{P} \times T}}} & (2)\end{matrix}$

The switching period T varies according to frequency modulation. Asshown in equation (2), the output power P_(o) will vary as the switchingperiod T varies. Therefore, an undesirable ripple signal will begenerated as the output power P_(o) varies.

An object of the present invention is to provide a switching controllerhaving switching frequency hopping to reduce the EMI for a power supply,in which the switching controller of the present invention will notgenerate the undesired ripple signal at the output of the power supply.

SUMMARY OF THE INVENTION

A switching controller having frequency hopping for power suppliesaccording to the present invention includes a pattern generator,receiving a clock signal and generating a digital pattern code inresponse to the clock signal. An oscillator produces an oscillationsignal to determine a switching frequency of a switching signal. Aprogrammable capacitor coupled to the oscillator is utilized to modulatethe switching frequency in response to the digital pattern code. Anattenuator is connected to a voltage feedback loop for attenuating afeedback signal, in which the feedback signal is utilized to control thepulse width of the switching signal and to control the output power ofthe power supply. A programmable resistor is connected to the attenuatorfor programming an attenuation rate of the attenuator in response to thedigital pattern code. The attenuation rate is increased as the switchingfrequency increases. The pulse width of the switching signal is thusreduced, which compensates the decrease of the switching period andkeeps the output power and output voltage of the power supply constant.

A method for generating a hopping frequency through a switchingcontroller is provided. The method comprises: generating a clock signal;generating a digital pattern code in response to said clock signalthrough a pattern generator; determining a switching frequency of aswitching signal through an oscillator, wherein said switching signal isutilized to regulate an output of a power supply, and wherein saidswitching signal is synchronized with said clock signal; and modulatingsaid switching frequency in response to said digital pattern codethrough a programmable capacitor coupled to said oscillator.

It is to be understood that both the foregoing general descriptions andthe following detailed descriptions are exemplary, and are intended toprovide further explanation of the invention as claimed. Still furtherobjects and advantages will become apparent from a consideration of theensuing description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention.

FIG. 1 shows a traditional power supply having an EMI filter.

FIG. 2 shows an embodiment of a switching controller having frequencyhopping according to the present invention.

FIG. 3 shows an embodiment of an oscillator according to the presentinvention.

FIG. 4 shows an embodiment of a pattern generator according to thepresent invention.

FIG. 5 shows another embodiment of the pattern generator according tothe present invention.

FIG. 6 shows a PWM control circuit.

DESCRIPTION OF THE EMBODIMENTS

FIG. 1 shows a traditional power supply having an EMI filter. A PWMcontroller 10 modulates the pulse width of a switching signal VPWM inresponse to a feedback signal VFB. The feedback signal VFB is obtainedfrom an optical-coupler 85. An operational amplifier 80 and a referencevoltage VREF form an error amplifier to drive the optical-coupler 85.Resistors 72, 73 and the error amplifier form a voltage feedback loop toregulate an output voltage VO of the power supply. A switching currentIP of a transformer 11 is converted to a switching-current signal VSthrough a sense resistor 30. The switching-current signal VS is providedto the PWM controller 10 for the pulse width modulation of the switchingsignal VPWM.

FIG. 2 is an embodiment of a switching controller having frequencyhopping according to the present invention. A clock generator 400generates a clock signal CK. A pattern generator 300 is utilized togenerate a digital pattern code [Mn . . . M1] in response to the clocksignal CK. An oscillator 200 is used for determining a switchingfrequency of the switching signal VPWM. The switching signal VPWM issynchronized with the clock signal CK. A programmable capacitor 290 iscoupled to the oscillator 200 for modulating the switching frequency inresponse to the digital pattern code [Mn . . . M1]. A resistor RA and aresistor RB develop an attenuator 500. The resistor RA is coupled to thevoltage feedback loop for attenuating the feedback signal VFB, in whicha first terminal of the resistor RA is supplied with the feedback signalVFB. The feedback signal VFB is utilized to control the pulse width ofthe switching signal VPWM via a PWM control circuit 600 and thereforecontrol the output power of the power supply. A second terminal of theresistor RA is connected to a first terminal of the resistor RB. Asecond terminal of the resistor RB is connected to a ground referencelevel. A voltage VB at the first terminal of the resistor RB isoutputted from an output of the attenuator 500. A programmable resistor100 is connected to the output of the attenuator 500 for programming anattenuation rate of the attenuator 500 in response to the digitalpattern code [Mn . . . M1]. The attenuation rate is increased wheneverthe switching frequency increases. The pulse width of the switchingsignal VPWM is thus reduced, which compensates the decrease in theswitching period and keeps the output power and output voltage of thepower supply constant.

The programmable resistor 100 comprises switching-resistor setsconnected to each other in parallel, in which the switching-resistorsets are formed by resistors R1, R2, . . . , Rn and switches S1, S2, . .. Sn. The switch S1 and the resistor R1 are connected in series. Theswitch S2 and the resistor R2 are connected in series. The switch Sn andthe resistor Rn are connected in series. The digital pattern code [Mn .. . M1] controls switches S1, S2, . . . Sn.

The programmable capacitor 290 comprises switching-capacitor setsconnected to each other in parallel, in which the switching-capacitorsets are formed by capacitors C1, C2, . . . Cn and switches X1,X2, . . .Xn. The switch X1 and the capacitor C1 are connected in series. Theswitch X2 and the capacitor C2 are connected in series. The switch Xnand the capacitor Cn are connected in series. The digital pattern code[Mn . . . M1] controls switches X1, X2, . . . Xn.

FIG. 3 shows an embodiment of the oscillator 200 according to thepresent invention. A current source I10 produces a charge current ICHG.A current source 120 produces a discharge current IDCHG.

A charge switch S31 is connected between the current source I10 and acapacitor C. A discharge switch S41 is connected between the capacitor Cand the current source I20. A comparator 210 has a positive inputsupplied with a threshold voltage VH. A negative input of the comparator210 is connected to the capacitor C. A comparator 220 has a negativeinput supplied with a threshold voltage VL. The threshold voltage VH ishigher than the threshold voltage VL. The positive input of thecomparator 220 is connected to the capacitor C. An output of a NAND gate230 generates an oscillation signal PLS to turn on/off the dischargeswitch S41. A first input of the NAND gate 230 is driven by an output ofthe comparator 210. Two inputs of a NAND gate 240 are respectivelyconnected to the output of the NAND gate 230 and an output of thecomparator 220. The output of the NAND gate 240 is connected to a secondinput of the NAND gate 230 and turns on/off the charge switch S31.

FIG. 4 shows an embodiment of the pattern generator 300 according to thepresent invention. A timer 310 generates a binary code [bn . . . b0] inresponse to the clock signal CK. A read-only-memory 320 generate thedigital pattern code [Mn . . . M1] in response to the binary code [bn .. . b0]. The address inputs of the read-only-memory 320 are driven bythe output of the timer 310.

FIG. 5 shows another embodiment of the pattern generator 300 accordingto the present invention. A plurality of registers 331, 332 . . . 335and a XOR gate 339 develop a linear shift register for generating alinear code in response to the clock signal CK. The inputs of the XORgate 339 determine the polynomials of the linear shift register anddecide the output of the linear shift register. Furthermore, the digitalpattern code [Mn . . . M1] can be adopt from the part of the linear codeto optimize the application.

FIG. 6 shows an embodiment of the PWM control circuit 600 according tothe present invention. The PWM control circuit 600 comprises acomparator 610, a D flip-flop 620 and an AND gate 630. The comparator610 is used to reset the D flip-flop 620. The voltage VB outputted fromthe attenuator 500 supplies a positive input of the comparator 610. Theswitching-current signal VS supplies a negative input of the comparator610. A D input of the D flip-flop 620 is pulled high by a supply voltageVCC. A clock input of the D flip-flop 620 is supplied with theoscillation signal PLS. A first input of the AND gate 630 is suppliedwith the oscillation signal PLS. A second input of the AND gate 630 isconnected to an output of the D flip-flop 620. An output of the AND gate630 generates the switching signal VPWM.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncovers modifications and variations of this invention provided they fallwithin the scope of the following claims and their equivalents.

1. A switching controller having frequency hopping, comprising: apattern generator, receiving a clock signal and generating a digitalpattern code in response to said clock signal; an oscillator, fordetermining a switching frequency of a switching signal, wherein saidswitching signal is utilized to regulate an output of a power supply,and wherein said switching signal is synchronized with said clocksignal; and a programmable capacitor, coupled to said oscillator formodulating said switching frequency in response to said digital patterncode.
 2. The switching controller having frequency hopping of claim 1,wherein said programmable capacitor comprises: switching-capacitor sets,connected to each other in parallel, wherein said switching-capacitorsets are formed by couples of switches and capacitors connected inseries respectively, wherein said switches are turned on/off by saiddigital pattern code.
 3. The switching controller having frequencyhopping of claim 1, wherein said oscillator comprises: a charge-currentsource, for producing a charge current; a discharge-current source, forproducing a discharge current; an osc capacitor, connected in parallelwith said programmable capacitor; an osc-charge switch, connectedbetween said charge-current source and said osc. capacitor; anosc-discharge switch, connected between said discharge-current sourceand said osc capacitor; a first comparator, wherein a positive input ofsaid first comparator is supplied with an first osc-threshold voltage, anegative input of said first comparator is connected to said osccapacitor; a second comparator, wherein a negative input of said secondcomparator is supplied with a second osc-threshold voltage, a positiveinput of said second comparator is connected to said osc capacitor, andwherein said first osc-threshold voltage is higher than said secondosc-threshold voltage; a first NAND gate, for producing an oscillationsignal for determining said switching frequency, wherein a first inputof said first NAND gate is driven by an output of said first comparator,and wherein an output of said first NAND gate turns on/off saidosc-discharge switch; and a second NAND gate, wherein two inputs of saidsecond NAND gate are respectively connected to said output of said firstNAND gate and an output of said second comparator, wherein an output ofsaid second NAND gate is connected to a second input of said first NANDgate, and wherein said output of said second NAND gate turns on/off saidosc-charge switch.
 4. The switching controller having frequency hoppingof claim 1, further comprising:. an attenuator, coupled to a voltagefeedback loop for attenuating a feedback signal, wherein said feedbacksignal is utilized to control the pulse width of said switching signal.5. The switching controller having frequency hopping of claim 4, whereinthe switching controller further comprises: a programmable resistor,connected to said attenuator for programming an attenuation rate of saidattenuator in response to said digital pattern code.
 6. A method forgenerating a hopping frequency through a switching controller,comprising: generating a clock signal; generating a digital pattern codein response to said clock signal through a pattern generator;determining a switching frequency of a switching signal through anoscillator, wherein said switching signal is utilized to regulate anoutput of a power supply, and wherein said switching signal issynchronized with said clock signal; and modulating said switchingfrequency in response to said digital pattern code through aprogrammable capacitor coupled to said oscillator.
 7. The method ofclaim 6, wherein said programmable capacitor comprises:switching-capacitor sets, connected to each other in parallel, whereinsaid switching-capacitor sets are formed by couples of switches andcapacitors connected in series respectively, wherein said switches areturned on/off by said digital pattern code.
 8. The method of claim 6,wherein said oscillator comprises: a charge-current source, forproducing a charge current; a discharge-current source, for producing adischarge current; an osc capacitor, connected in parallel with saidprogrammable capacitor; an osc-charge switch, connected between saidcharge-current source and said osc capacitor; an osc-discharge switch,connected between said discharge-current source and said osc capacitor;a first comparator, wherein a positive input of said first comparator issupplied with an first osc-threshold voltage, a negative input of saidfirst comparator is connected to said osc capacitor; a secondcomparator, wherein a negative input of said second comparator issupplied with a second osc-threshold voltage, a positive input of saidsecond comparator is connected to said osc capacitor, and wherein saidfirst osc-threshold voltage is higher than said second osc-thresholdvoltage; a first NAND gate, for producing an oscillation signal fordetermining said switching frequency, wherein a first input of saidfirst NAND gate is driven by an output of said first comparator, andwherein an output of said first NAND gate turns on/off saidosc-discharge switch; and a second NAND gate, wherein two inputs of saidsecond NAND gate are respectively connected to said output of said firstNAND gate and an output of said second comparator, wherein an output ofsaid second NAND gate is connected to a second input of said first NANDgate, and wherein said output of said second NAND gate turns on/off saidosc-charge switch.
 9. A switching controller having frequency hopping,comprising: a pattern generator, receiving a clock signal and generatinga digital pattern code in response to said clock signal; an oscillator,for determining a switching frequency of a switching signal, whereinsaid switching signal is utilized to regulate an output of a powersupply; and a frequency modulating circuit, coupled to said oscillatorfor modulating said switching frequency in response to said digitalpattern code.
 10. The switching controller having frequency hopping ofclaim 9, wherein said switching signal is substantially synchronizedwith said clock signal.
 11. The switching controller having frequencyhopping of claim 9, wherein the frequency modulating circuit is aprogrammable capacitor.
 12. The switching controller having frequencyhopping of claim 11, wherein said programmable capacitor comprises:switching-capacitor sets, connected to each other in parallel, whereinsaid switching-capacitor sets are formed by couples of switches andcapacitors connected in series respectively, wherein said switches areturned on/off by said digital pattern code.
 13. The switching controllerhaving frequency hopping of claim 9, further comprising: an attenuator,coupled to a voltage feedback loop for attenuating a feedback signal,wherein said feedback signal is utilized to control the pulse width ofsaid switching signal.
 14. The switching controller having frequencyhopping of claim 13, wherein the switching controller further comprises:a programmable resistor, connected to said attenuator for programming anattenuation rate of said attenuator in response to said digital patterncode.